1. Field of the Invention
The present invention relates to a control apparatus for controlling a lock-up clutch provided on an automotive vehicle, and more particularly to techniques for improving fuel economy of a lean-burn engine provided with a turbocharger (turbo-supercharger), by holding the lock-up clutch in an engaged or slipping state in a relatively wide range or area of running condition of the vehicle while the engine is placed in a relatively stable combustion state. The present invention is also concerned with a control apparatus for controlling such a lean-burn engine provided with a turbocharger.
2. Discussion of Related Art
For controlling a lean-burn engine provided with a turbocharger so as to operate the engine with a relatively high degree of fuel economy in a wide range or area of running condition of the vehicle, it has been proposed to determine the manner of controlling the speed ratio of an automatic transmission so as to enable the vehicle to run in a steady state while the engine is operated at a relatively low speed in its turbocharging state. An example of such a transmission control apparatus is incorporated in a vehicle control apparatus disclosed in JP-A-7-156691. This transmission control apparatus permits the engine to operate at a low speed in the turbocharging state, that is, with high fuel economy, in a comparatively wide range of running condition of the vehicle, so that the fuel economy of the engine can be significantly enhanced.
In the automotive vehicle equipped with the vehicle control apparatus described above, the speed ratio of the automatic transmission is controlled to enlarge the range or area of the running condition of the vehicle in which the engine is operated at a low speed in its turbocharging state with a relatively high degree of stability of its combustion state. However, the range of the vehicle running condition in which the lock-up clutch is placed in its engaged state is not considered to improve the fuel economy of the lean-burn engine with the turbocharger. In this respect, there is still a room for improvement of the lean-burn engine provided with the turbocharger.
In an automotive vehicle including a lean-burn engine provided with a turbocharger, the intake air quantity can be generally increased by operating the turbocharger under a non-full-load condition, so that the engine can be a lean-burn state under the non-full-load condition. Further, the engine in the turbocharging state can be operated with a comparatively high degree of stability in its combustion state, so that the upper limit of the air/fuel ratio of the air-fuel mixture can be increased. However, increasing the air/fuel ratio tends to cause undesirable vibration or surge of the engine, which deteriorates the running comfort of the vehicle. In this respect, there is a limitation in the upper limit of the air/fuel ratio of the air-fuel mixture, so that the lean-burn operation of the engine is limited to prevent the surge of the engine.
JP-A-11-117785 proposes an engine control technique for maximizing the upper limit of the air/fuel ratio of the air-fuel mixture in the lean-burn operation of the engine under a non-full-load condition. According to this engine control technique, the air/fuel ratio of the air-fuel mixture is controlled by changing the intake air quantity relative to the fuel amount, by controlling the turbocharging pressure to be established by the turbocharger of the engine. The turbocharging pressure is controlled so that the actual degree of surge of the engine coincides with a predetermined target value.
However, the known engine control technique indicated above does not take into account an influence of the operating state of the lock-up clutch on the surge of the engine. Accordingly, the known engine control technique does not permit the lean-burn operation of the engine with a sufficiently high air/fuel ratio of the air-fuel mixture, in relation to the specific operating state of the lock-up clutch.
It is therefore a first object of the present invention to provide a control apparatus for controlling a lock-up clutch provided on an automotive vehicle, which apparatus permits a further improvement in the fuel economy of a lean-burn engine of the vehicle provided with a turbocharger.
It is a second object of this invention to provide a control apparatus for controlling a lean-burn engine of an automotive vehicle provided with a turbocharger, which permits the engine to be operated in a lean-burn state with the air/fuel ratio of the air-fuel mixture being variable over a sufficiently wide range, in relation to the specific operating state of the lock-up clutch, while presenting or reducing the surge of the engine.
The first object indicated above may be achieved according to a first aspect of this invention, which provides an apparatus for controlling a lock-up clutch disposed in a drive system of an automotive vehicle which includes a lean-burn engine provided with a turbocharger, the apparatus being arranged to place the lock-up clutch in a fully engaged state while a running condition of the automotive vehicle is in a predetermined engaging area of the lock-up clutch, the apparatus being characterized by comprising engaging-area changing means for changing the engaging area of the lock-up clutch, on the basis of a turbocharging pressure established by the turbocharger.
In the lock-up clutch control apparatus constructed according to the first aspect of this invention described above, the engaging-area changing means is arranged to change the engaging area of the lock-up clutch, on the basis of the turbocharging pressure established by the turbocharger, so that the engaging area can be enlarged with an increase in the turbocharging pressure while the lean-burn engine provided with the turbocharger is placed in the turbocharging lean-burn state in which the combustion state of the engine is relatively stable with the turbocharging pressure being relatively high, whereby the torque transmission efficiency of the engine can be maximized so as to improve the fuel economy of the engine.
In one preferred form of the lock-up clutch control apparatus according to the first aspect of the present invention, the engaging-area changing means is operable to enlarge the engaging area of the lock-up clutch with an increase in said turbocharging pressure. In this form of the invention wherein the engaging-area changing means enlarges the engaging area of the lock-up clutch as the turbocharging pressure is increased, the torque transmission efficiency of the engine can be improved to thereby enhance the fuel economy of the engine as the turbocharging pressure is raised in the turbocharging lean-burn state of the engine in which the combustion state is relatively stable.
In another preferred form of the first aspect of the invention, the apparatus further comprises engine torque control means for controlling the lean-burn engine such that an output torque of the lean-burn engine is held constant, on the basis of the turbocharging pressure and an air/fuel ratio of an air-fuel mixture introduced in the engine. In this form of the lock-up clutch control apparatus, the air/fuel ratio of the air-fuel mixture is increased with an increase in the turbocharging pressure, for holding the output torque of the engine constant, so that the fuel economy is further improved while the engine is operated under a low-speed high-load condition, that is, operated at a relatively low speed and in the turbocharging state.
The second object indicated above may be achieved according to a second aspect of this invention, which provides an apparatus for controlling a lock-up clutch disposed in a drive system of an automotive vehicle which includes a lean-burn engine provided with a turbocharger, the apparatus being arranged to place the lock-up clutch in a slip control state while a running condition of the automotive vehicle is in a predetermined slip control area of the lock-up clutch, the apparatus being characterized by comprising slip-control-area changing means for changing the slip control area of the lock-up clutch, on the basis of a turbocharging pressure established by the turbocharger.
In the lock-up clutch control apparatus constructed according to the second aspect of this invention, the slip-control-area changing means is arranged to change the slip control area of the lock-up clutch, on the basis of the turbocharging pressure established by the turbocharger, so that that the slip control area of the lock-up clutch can be enlarged with an increase in the turbocharging pressure while the lean-burn engine provided with the turbocharger is placed in the turbocharging lean-burn state in which the combustion state of the engine is relatively stable with the turbocharging pressure being relatively high, whereby the torque transmission efficiency of the engine can be maximized so as to improve the fuel economy of the engine.
In one preferred form of the lock-up clutch control apparatus according to the second aspect of this invention, the slip-control-area changing means is operable to enlarge the slip control area of the lock-up clutch with an increase in the turbocharging pressure. In this preferred form of the apparatus wherein the slip-control-area changing means enlarges the slip control area of the lock-up clutch as the turbocharging pressure is increased, the torque transmission efficiency of the engine can be improved to thereby enhance the fuel economy of the engine as the turbocharging pressure is raised in the turbocharging lean-burn state of the engine in which the combustion state is relatively stable.
In another preferred form of the second aspect of the invention, the lock-up clutch control apparatus further comprises engine torque control means for controlling the lean-burn engine such that an output torque of the lean-burn engine is held constant, on the basis of the turbocharging pressure and an air/fuel ratio of an air-fuel mixture introduced in the engine. In this form of the lock-up clutch control apparatus, the air/fuel ratio of the air-fuel mixture is increased with an increase in the turbocharging pressure, for holding the output torque of the engine constant, so that the fuel economy is further improved while the engine is operated under a low-speed high-load condition, that is, operated at a relatively low speed and in the turbocharging state.
The second object indicated above may be achieved according to a third aspect of the present invention, which provides an apparatus for controlling a lean-burn engine provided with a turbocharger disposed in a drive system of an automotive vehicle which includes a lock-up clutch, the lean-burn engine being operated with an air-fuel mixture whose air/fuel ratio for a given value of a turbocharging pressure to be established by the turbocharger is variable in a predetermined air/fuel ratio range, the apparatus being characterized by comprising air/fuel-ratio-range changing means for changing the air/fuel ratio range, on the basis of at least one of an operating state and a slipping state of the lock-up clutch.
In the present lean-burn engine control apparatus according to the third aspect of this invention described above, the air/fuel-ratio-range changing means is arranged to change the air/fuel ratio range for a given value of the turbocharging pressure of the lean-burn engine, on the basis of the operating state and/or the slipping state of the lock-up clutch. Where the air/fuel-ratio-range changing means is arranged to change the air/fuel ratio range on the basis of at least the operating state (fully engaged state, fully released state or slip control state) of the lock-up clutch, the upper limit of the air/fuel ratio range can be made relatively high when the lock-up clutch is placed in the fully released state or a state near the fully released state. Accordingly, the lean-burn engine provided with the turbocharger can be operated in a lean-burn state with the air/fuel ratio of the air-fuel mixture being variable over a sufficiently wide range, in relation to the operating state of the lock-up clutch, while presenting or reducing the surge of the engine.
Where the air/fuel-ratio-range changing means is arranged to change the air-fuel ratio range on the basis of at least the slipping state of the lock-up clutch, the upper limit of the air/fuel ratio range can be increased depending upon the slipping state or slip ratio of the lock-up clutch. Thus, the present arrangement permits the lean-burn engine to be operated in a lean-burn state with the air/fuel ratio being variable over a sufficiently wide range, in relation to the slipping state of the lock-up clutch, while reducing the surge of the engine.
In one preferred form of the lean-burn engine control apparatus according to the third aspect of this invention, the air/fuel-ratio-range changing means changes the air/fuel ratio range on the basis of both of the operating state and the slipping state of the lock-up clutch.
In the above preferred form of the lean-burn engine control apparatus wherein the air/fuel ratio range is changed on the basis of the operating state and the slipping state of the lock-up clutch, the upper limit of the air/fuel ratio range can be made relatively high when the lock-up clutch is in the fully released state or a state near the fully released state, and can be increased with an increase in the slip ratio of the lock-up clutch. Accordingly, the lean-burn engine provided with the turbocharger can be operated in a lean-burn state with the air/fuel ratio of the air-fuel mixture being variable over a sufficiently wide range, in relation to the operating state of the lock-up clutch, while presenting or reducing the surge of the engine.
In another preferred form of the lean-burn engine control apparatus, the air/fuel-ratio-range changing means changes the air/fuel ratio range such that the air/fuel ratio range is wider when the lock-up clutch is placed in a fully released state than when the lock-up clutch is not placed in the fully released state. In this form of the lean-burn engine control apparatus, the air/fuel ratio range is wider in the fully released state of the lock-up clutch, than in the fully engaged state, so that the engine is less likely to suffer from a surge or vibration in the fully released state of the lock-up clutch, since the surge is more likely to be absorbed by the lock-up clutch in the fully released state than in the fully engaged state. Accordingly, the engine can be operated in a lean-burn state with a sufficiently high air/fuel ratio of the air-fuel mixture in the fully released state of the lock-up clutch.
In a further preferred form of the lean-burn engine control apparatus, the air/fuel-ratio-range changing means includes air/fuel-ratio changing means for increasing the air/fuel ratio of the air-fuel mixture with an increase in the turbocharging pressure. In a still further preferred form of the lean-burn engine control apparatus, the air-fuel-ratio-range changing means enlarges the air/fuel ratio range with an increase in the turbocharging pressure. In these preferred forms of the lean-burn engine control apparatus, the combustion state of the lean-burn engine can be stabilized when the turbocharging pressure is relatively high, thereby permitting significant reduction of the surge of the engine during a lean-burn operation with a relatively high air/fuel ratio of the air-fuel mixture. Thus, the present lean-burn engine control apparatus assures a lean-burn operation of the engine with a reduced degree of surge.
In a yet further preferred form of the lean-burn engine control apparatus, the air/fuel-ratio-range changing means enlarges the air/fuel ratio range with an increase in a slip ratio of the lock-up clutch. According to this control apparatus, the engine is less likely to suffer from a surge or vibration when the slip ratio of the lock-up clutch is relatively high, since the surge is more likely to be absorbed by the lock-up clutch when the slip ratio is relatively high, so that the engine can be operated in a lean-burn state with a sufficiently high air/fuel ratio of the air-fuel mixture when the lock-up clutch has a relatively high slip ratio.